a) Field of the Invention
A great variety of types and constructions of line-shaped weakened portions are introduced in a wide variety of materials for the production of predetermined breaking lines. Probably the most frequently used type is a perforation line penetrating the material. The geometry and dimensions of the alternating through-holes and webs are selected depending on the strength of the material, the planned use and the production technique. Continuous perforation lines are often applied, for example, in the manufacture of packaging products, in the production of continuous rolls of forms or blocks of postage stamps. In the applications mentioned above, the parts of the material adjoining the perforation line are generally effectively separated from one another by deliberate application of force in the direction of the perforation line. Consequently, the straight line is compulsorily the optimal line shape for carrying out the severing with a fluid motion and with uniform force. As a rule, the fact that the perforation line is visible by the naked eye is advantageous and obviates additional marking of the line-shaped weakened portions, e.g., by printed marks.
Another type of line-shaped weakening is the embossed line which, however, allows only a slight weakening of material and is limited to a few materials such as paper. The lines are also straight lines in this instance and are visible with the naked eye.
The production of line-shaped weakened portions by means of deep cuts is used primarily when the predetermined breaking line should be invisible, e.g., in an airbag cover. The airbag can be arranged under the dashboard, below the steering wheel hub or door panel, for example. For the purposes of the following discussion, all conceivable flat bodies behind which or under which the airbag can be located are referred to as airbag sheathing. By airbag cover is meant the surface in the airbag sheathing which is removed or destroyed during the unfolding of the airbag so that an opening is formed for the airbag to pass through. For cosmetic reasons, however, it is desirable that this airbag covering in the airbag sheathing is invisible to the naked eye. Nevertheless, for the sake of completeness, the known solutions in which the airbag covering is realized in a manner other than by deep cuts in the airbag sheathing will be mentioned herein.
b) Description of the Related Art
It is known that manufacturers create the airbag cover in that the airbag sheathing is cut out on the desired predetermined breaking line and is then closed with a cover. A solution of this kind is sometimes considered unappealing in a cosmetic sense and is very cost-intensive.
Other manufacturers cut the airbag cover in the airbag sheathing in the desired contour and then cover the airbag sheathing and the airbag cover which is reinserted therein with a thin sheet whose strength and material characteristics are so dimensioned that this sheet tears in case of need. The disadvantage in this solution consists in that the thin sheet is located in the created cut gap, so that the contour of the airbag cover is clearly visible. Further, the desired breaking resistance is only possible by appropriate selection of the sheet and its material characteristics and strength. When a conventional wear-resistant or long-wearing sheet is used, the tearing behavior is correspondingly poor.
The production of the airbag cover by arranging line-shaped weakened portions in the otherwise finished airbag sheathing is more economical than the production process mentioned above. Various processes are known for producing line-shaped weakened portions in the airbag sheathing by cutting and will be considered in more detail hereinafter.
As was already mentioned, it is also known to arrange line-shaped weakened portions by means of incisions in the otherwise finished airbag sheathing.
There are airbag sheathings formed of one-layer plastic material or of two-layer laminated material, e.g., with an inner layer of low-density polyethylene foam and an outer layer of high-density polyethylene foam as is described in U.S. Pat. No. 4,120,516. In order to produce line-shaped weakened portions in this laminated material, the laminated material is cut from the inside out by means of cutting blades. The cut is accordingly made through the low-density foam into the relatively high-density foam. The high-density foam can be supported on a supporting surface so that pressure can be exerted by the cutting blades without leading to problems in producing the cuts and without the pressure producing press marks in the outer skin of the outer layer which will become visible subsequently.
For passenger vehicles in the upper price classes, the material of the airbag sheathing is often a reverse sandwich construction. The inner layer which gives shape and stability to the airbag sheathing is a plastic layer or resin fiber layer of high density. To this layer is applied a foamed material layer of low density which is covered by plastic sheeting. In order to make a deep cut in such a material construction according to the method described in U.S. Pat. No. 4,120,516, the foamed material layer would have to be compressed. This would make it considerably more difficult to produce a cut of definite depth. Further, permanently visible marks could be made on the plastic sheeting side. DE 44 09 405 proposes a method for producing an airbag cover in which the airbag sheathing is formed of an inner shaped layer of relatively hard polymer material and an outer layer of relatively soft polymer material. For this purpose, a cut line is to be inserted proceeding from the inner shaped layer which completely penetrates the inner shaped layer without exerting pressure on the airbag sheathing. The cut is advantageously made by means of a laser and also extends along a determined depth in the outer layer.
In order to allow the airbag to unfold reliably and within the shortest time in case of need, it is important that the required breaking force is constant and as small as possible along the entire line of weakened material. The breaking force to be applied is determined substantially by the strength of the material itself and by the material strength in the area of the line-shaped weakened portion. Therefore, in a sandwich construction which terminates on the driver's compartment side in a long-wearing fixed sheet, it is necessary to introduce the line-shaped weakening into the sheet. However, this results in high thermal loading, heating and consequently softening of the sheet. As a result, the sheet might possibly settle in the cut gap so that the line-shaped weakening is visible as a recess from the driver's compartment side. The same thing occurs in the case of thinner or soft sheets, also because the deep cut made by means of a laser only extends below the sheeting.
Often the course of the predetermined breaking line is identical to the contour of the airbag cover, wherein the line-shaped weakening forms the circumference of the airbag cover. The line-shaped weakening can then be a rectangle, for example, or its configuration can be adapted to the shape of the airbag sheathing, as is shown, for example, in DE 44 09 405, to the size and shape of a steering wheel hub.
EP 0648 646 A1 discloses an airbag cover in which the line-shaped weakening does not conform to the circumference of the airbag cover as described above, but rather in which the line-shaped weakening runs from the corners of the airbag cover to the surface center of gravity. When the airbag opens, the airbag cover is therefore torn from its surface center of gravity. As follows from the description of the prior art in EP 0648 646 A1, other "patterns" of line-shaped weakened portions which determine the predetermined breaking contour are also known in airbag covers. Also, in the known "H" or "X" patterns mentioned herein, the line-shaped weakened portions like the predetermined breaking lines are essentially straight-line deep cuts. Regardless of the production technique and patterns of deep cuts, there is a risk that the deep cuts will be visible to the passengers of the vehicle due to thermal loading during production and due to long-term influences (temperature, material aging, light, etc.).